Streptococcus species, of which there are a large variety that cause infections in domestic animals and man, are often grouped according to Lancefield's groups. Typing, according to Lancefield, occurs on the basis of serological determinants or antigens that are, among others, present in the capsule of the bacterium and, thus, allows for an approximate determination. Often, bacteria from a different group show cross-reactivity with each other, while other Streptococci cannot be assigned a specific group-determinant at all. Within groups, further differentiation is often possible on the basis of serotyping. These serotypes further contribute to the large antigenic variability of Streptococci, a fact that creates an array of difficulties within diagnosis of and vaccination against Streptococcal infections.
Lancefield group A Streptococcus (GAS, Streptococcus pyogenes) are common with children and cause nasopharyngeal infections and complications thereof. Among animals, cattle are susceptible to GAS, and mastitis is often found.
Lancefield group B Streptococcus (GBS) are most often seen with cattle and cause mastitis. However, human infants are susceptible as well, often with fatal consequences. Group B streptococci (GBS) constitute a major cause of bacterial sepsis and meningitis among human neonates born in the United States and Western Europe and are emerging as significant neonatal pathogens in developing countries.
Lancefield group C infections, such as those with S. equi, S. zooepidemicus, S. dysgalactiae, and others are mainly seen with horse, cattle and pigs, but can also cross the species barrier to humans.
Lancefield group D (S. bovis) infections are found with all mammals and some birds, sometimes resulting in endocarditis or septicemia.
Lancefield groups E, G, L, P, U and V (S. porcinus, S. canis, S. dysgalactiae) are found with various hosts and cause neonatal infections, nasopharyngeal infections or mastitis.
Within Lancefield groups R, S, and T (and with ungrouped types), S. suis is found and is an important cause of meningitis, septicemia, arthritis and sudden death in young pigs. Incidentally, it can also cause meningitis in man.
Ungrouped Streptococcus species, such as S. mutans, causes caries with humans, S. uberis, causes mastitis with cattle, and S. pneumonia, causes major infections in humans, and Enterococcus faecalis and E. faecium, further contribute to the large group of Streptococci. Streptococcus pneumoniae (the pneumococcus) is a human pathogen that causes invasive diseases, such as pneumonia, bacteremia, and meningitis.
Little is known about the pathogenesis of the disease caused by Streptococci. Various cellular components, such as muramidase-released protein (MRP), extracellular factor (EF) and cell membrane-associated proteins, fimbriae, hemagglutinins, and hemolysin, have been suggested as virulence factors. However, the precise role of these protein components in the pathogenesis of the disease remains unclear. It is, however, known and generally accepted that the polysaccharidic capsule of various Streptococci and other gram-positive bacteria plays an important role in pathogenesis. The capsule enables these microorganisms to resist phagocytosis and is, therefore, regarded as an important virulence factor or marker.
In particular, Streptococcus suis is an important cause of meningitis, septicemia, arthritis and sudden death in young pigs. It can also cause meningitis in man. Attempts to control the disease are hampered by the lack of sufficient knowledge about the pathogenesis of the disease and the lack of effective vaccines and sensitive diagnostic methods.
So far, 35 serotypes of S. suis have been described. Virulence of S. suis can differ within and among serotypes. Worldwide, S. suis serotype 2 is the most frequently isolated serotype. Within S. suis serotype 2, pathogenic, weak-pathogenic and non-pathogenic strains can be found. The pathogenic strains cause severe clinical signs of disease in pigs and large numbers of bacteria can be re-isolated from the central nervous system (CNS) and the joints after experimental infection. The weak-pathogenic strains cause only mild clinical signs of disease and infrequently bacteria can be re-isolated from the CNS and the joints after experimental infection. The non-pathogenic strains are completely avirulent in young pigs after experimental infection.
The 136-kDa muramidase-related protein (MRP) and the 110-kDa extracellular factor (EF) are generally considered as important virulence markers for S. suis serotype 2 strains isolated in Europe and the United States. However, differences in virulence between pathogenic, weak-pathogenic and non-pathogenic strains cannot exclusively be explained by differences in their MRP and EF expression patterns. In addition, it is known that the capsule of Streptococcus suis serotype 2 is an important virulence factor. However, since pathogenic, weak-pathogenic and non-pathogenic strains seem to be fully encapsulated after growth in vitro and in vivo, it is not likely that the level of encapsulation of these fully encapsulated strains is associated with their difference in virulence.